The present invention relates to semiconductor optical devices and a method of manufacturing them and, more particularly, to a semiconductor optical device having two sides of an active region buried in a semi-insulating crystal and a method of manufacturing the device.
A semi-insulating buried heterostructure (SIBH) having a semi-insulating layer as a buried layer is used for a semiconductor optical device such as a semiconductor laser diode or semiconductor optical modulator. It is known that when this structure is used for such a device, lower device capacitance and higher speed modulation can be realized than when a p-n buried structure is used. For this reason, a semi-insulating buried heterostructure is indispensable to semiconductor optical modulators and semiconductor optical devices used for a high capacity optical transmission system.
When such a semi-insulating buried heterostructure is used, many defects originating from damage due to a mesa process and an impurity used in a regrowth process exist in a regrowth interface in a burying growth process. This produces a leakage current when the device operates. In a semiconductor laser diode, this causes an increase in threshold current, a decrease in optical output efficiency, a deterioration in temperature characteristics, and the like.
In addition, a semiconductor crystal doped with iron (Fe) is conventionally used for such a semi-insulating buried heterostructure. If, however, iron (Fe) is used as a dopant, interdiffusion of iron (Fe) as a dopant for a semi-insulating buried layer and zinc (Zn) as a dopant for a p-cladding layer and p-contact layer of the device occurs at the interface between the semi-insulating buried layer and the device. As a consequence, as zinc is diffused into the buried layer, the characteristics of the device deteriorate, resulting in a deterioration in modulation characteristics.
In addition, Zn moved to the interstitial position between lattices by the above interdiffusion is also diffused into the active layer having an interface with the buried layer, resulting in a decrease in the optical output efficiency of the active layer.
It is known that the above interdiffusion is not limited to the case wherein Zn is used as a p-impurity, and other p-impurities such as Be, Cd, and Mg also cause interdiffusion with Fe.
As shown in FIG. 5, a technique of solving such a problem by inserting an Fe diffusion preventing layer 36 between a mesa stripe (MS) and an Fe-doped InP buried layer 37 is disclosed (Japanese Patent Laid-Open No. 9-214045). Referring to FIG. 11, reference numeral 31 denotes a semiconductor substrate; 32, a buffer layer; 33, an active layer; 34, a cladding layer; 35, a contact layer, and 36, an Fe diffusion preventing layer.
Recently, it has been found that in a semi-insulating semiconductor crystal doped with ruthenium (symbol of element: Ru), almost no interdiffusion occurs between Ru and Zn.
As shown in FIG. 6, the manufacture of a semiconductor laser using Ru-doped semi-insulating buried layers has been reported (xe2x80x9cA. Dadger et.al, Applied Physics Letters Vol. 73, No. 26 pp. 3878-3880 (1998)xe2x80x9d, xe2x80x9cA. Van Geelen et. at., 11th International Conference on Indium Phosphide and Related materials TuB 1-2 (1999)xe2x80x9d).
It is an object of the present invention to provide a semiconductor optical device which has a structure that can control diffusion of an impurity contained in a layer constituting a mesa stripe into a semi-insulating buried layer formed on two sides of the mesa stripe, and a method of manufacturing the device.
In order to achieve the above object, the present inventor has found that impurity diffusion into a buried layer can be controlled if a semi-insulating buried layer arranged on two sides of a layer forming a mesa stripe including an n-cladding layer, active region, and p-cladding layer is constituted by a diffusion enhancement layer which is adjacent to the mesa-stripe-like multilayer structure and enhances diffusion of a p-impurity, and a diffusion suppression layer which is adjacent to the diffusion enhancement layer and contains a semi-insulating impurity that suppresses diffusion of the p-impurity.
According to an aspect of the present invention, there is provided a semiconductor optical device comprising, on a semiconductor substrate, a mesa-stripe-like multilayer structure constituted by at least an n-cladding layer, an active region formed from an active layer or a photoabsorption layer, and a p-cladding layer, and a buried layer in which two sides of the multilayer structured are buried using a semi-insulating semiconductor crystal, the buried layer including a diffusion enhancement layer which is adjacent to the mesa-stripe-like multilayer structure and enhances diffusion of a p-impurity, and a diffusion suppression layer which is adjacent to the diffusion enhancement layer and suppresses diffusion of a p-impurity.
According to another aspect of the present invention, there is provided a semiconductor optical device comprising a semiconductor laser including, on a semiconductor substrate, a mesa-stripe-like multilayer structure constituted by at least an n-cladding layer, an active region formed from an active layer, and a p-cladding layer, and a buried layer in which two sides of the multilayer structured are buried using a semi-insulating semiconductor crystal, the buried layer including a diffusion enhancement layer which is adjacent to the mesa-stripe-like multilayer structure and enhances diffusion of a p-impurity, and a diffusion suppression layer which is adjacent to the diffusion enhancement layer and suppresses diffusion of a p-impurity, and an electroabsorption optical modulator including, on a semiconductor substrate, a mesa-stripe-like multilayer structure constituted by at least an n-cladding layer, an active region formed from a photoabsorption layer, and a p-cladding layer, and a buried layer in which two sides of the multilayer structured are buried using a semi-insulating semiconductor crystal, the buried layer including a diffusion enhancement layer which is adjacent to the mesa-stripe-like multilayer structure and enhances diffusion of a p-impurity, and a diffusion suppression layer which is adjacent to the diffusion enhancement layer and suppresses diffusion of a p-impurity, wherein a monolithically integrated light source including the semiconductor laser and the electroabsorption optical modulator is formed.
According to still another aspect of the present invention, there is provided a method of manufacturing a semiconductor optical device, comprising the step of forming, on a semiconductor substrate, a mesa-stripe-like multilayer structure constituted by at least an n-cladding layer, an active region formed from an active layer or a photoabsorption layer, and a p-cladding layer, the step of processing the multilayer structure into a mesa stripe, and the step of forming a buried layer by burying two sides of the multilayer structured in a semi-insulating semiconductor crystal, the step of forming the buried layer including the step of forming a diffusion enhancement layer which is adjacent to the mesa-stripe-like multilayer structure and enhances diffusion of a p-impurity, and the step of forming a diffusion suppression layer which is adjacent to the diffusion enhancement layer and suppresses diffusion of a p-impurity.
According to still another aspect of the present invention, there is provided a method of manufacturing a semiconductor optical device constituting monolithically integrated semiconductor laser and optical modulator on a semiconductor substrate, the step of forming the semiconductor optical device including the step of forming, on a semiconductor substrate, a mesa-stripe-like multilayer structure constituted by at least an n-cladding layer, an active region formed from optically coupled active layer and photoabsorption layer, and a p-cladding layer, and the step of forming a buried layer in which two sides of the multilayer structured are buried using a semi-insulating semiconductor crystal, the step of forming the buried layer including the step of forming a diffusion enhancement layer which is adjacent to the mesa-stripe-like multilayer structure and enhances diffusion of a p-impurity, and the step of forming a diffusion suppression layer which is adjacent to the diffusion enhancement layer and suppresses diffusion of a p-impurity.